INTERACTION OF SURFACTANTS AND KERATINS 49 electrostatics of the processes that occur when a freely floating ion in water (i.e., in a high dialectic constant medium) approaches an interface with a low dielectric constant medium such as a protein. According to electrostatic theory, an electric charge embedded in a high dielectric constant medium induces a repulsive force as it approaches an interface with a low dielectric constant continuum. The magnitude of this repulsive force can be calculated by assuming that a mirror image of the approaching ion exists on the other side of the interface (Figure 7). Using simple coulombic calculations, the repulsive electrostatic energy can be obtained from this type of model. This electrostatic repulsion of the mirror images is generally sufficient to overcome the attractive hydrophobic bonds and is sufficient to prevent the binding of an ionic species to a binding site that does not have a fixed oppositely charged ionic component (3). The situation is different, however, when the binding side is composed of a fixed charge with a hydrophobic region around it. In this case, the mirror image forces of the fixed charge and of the floating charge will cancel out and binding of the surfactant molecule with take place (Figure 8). Further verification for the mirror image effect as a reason for preventing the binding of charged species to proteins on noncharged sites, can also be obtained by examining the binding of divalent ions to proteins. Contrary to what is expected on the grounds of electrostatic attraction, it was found that a divalent ion will bind to a lesser extent to a protein than a monovalent ion of a similar structure and size. Medley (9) compared the titration curves of two dyes, Acid Orange 7 and Acid Orange 10, the structures of which only differed in as far as one of the dyes contained an additional sulfonic group attached to its molecular skeleton. (For formulae see Figure 9.) For the same dye concentration and pH, tl' divalent dye showed a lower uptake on wool than the monovalent species, in agreement with the theoretical predictions based on mirror image repulsion theory (Figure 10). In conclusion, it can be stated that both theoretical considerations and the experimental results indicate that charged surfactant molecules, unless bound by an oppositely charged group of the protein, have more difficulties in penetrating keratins than their uncharged counterparts. OH Figure 9. OH \so Formulae of Acid Orange 7 and Acid Orange 10. (a) C.I. Acid Orange 7 Naphthalene Orange G, or N.O.G. (b) C.I. Acid Orange 10 Napthalene Orange 2GS, or 2GS. (Reproduced with permission from reference 9.)

50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS O 0.8 0.2 0 I ! 2 3 Z, 5 - LOG (DYE ACID ACTIVITY) Figure 10. Titration curves of keratin with Acid Orange 7 (I) and Acid Orange 10 (II). (Reproduced with permission from reference 9.) IV. THE ROLE OF THE DIFFUSION PROCESSES IN DETERGENT-KERATIN INTERACTIONS The previous sections dealt with the uptake of detergents by keratins essentially from a thermodynamic point of view, i.e., it attempted to define the factors governing the uptakes at equilibria after exposures of very long times. When discussing effects of detergents under practical treatment conditions however, the rates of the surfactant uptake processes also become important. Since cosmetic treatments are generally of relatively short durations, i.e., last about 10-15 min, the detergents absorbed in the tissues will not reach a uniform, equilibrium distribution by the end of the treatment time. On the contrary, in most instances sharp concentration gradients of the surfactants will exist with the high values of concentration near the tissue surfaces exposed to the treatment solutions. As we shall discuss in the following sections of the paper, the properties of the keratinous tissues depend not only on the total amounts of surfactants present, but also on their spatial distribution which is determined by the